;36 



KNOWLEDGE • 



[Nov. 11, 1881. 



if, in your next niimbpr, it wcro puintod out at what hour, and in 

 what iliix'i'tiun, iutonding obscrvorH shuulil look out for tho Norcmbpr 

 raotoiirH — thot is, as proposed in tho c-urrcnt number, uso " plain un- 

 toc'iniciil hinguoge" ; and mny I further suggest that (making uso 

 of your own two typical words), if you write about small-faced 

 or jiing-hcaded men, you would cnmhino with them tho technical 

 word : c.'j., " the race was dolichocephalic, i.e., long-headed." By 

 thi.H moans }-our readers will understand tho technical terms, without 

 referring to their lexicons, when they come across them in more 

 advanced books. 



London, Nov. 9, 1881. A. T. C. 



[The meteors of November 13-lt will hardly be seen this year. 

 If seen at nil, it will be after midnight, and they may equally well 

 he looked for in all parts of tho sky. — Eu.] 



TUE EARTH'S INCLINATION. 

 [9] — It may seem a ridiculous question to ask you to answer, but 

 I can find no satisfaction in scientific primers. Will you kindly 

 toll mo, simply and clearly, the reason of this : — The earth's axis is 

 at an inclination of 2.3" (?) to the sun. Very good. Now suppose 

 tho northern hemisphere be nearer that body, if the earth should 

 spin around itself and the sun, why, when it gets on the opposite 

 side, will the northern hemisphere be turned from it ? Does the 

 bottom of the body move out [? move ne;xrer], while the top 

 rOLodes in its annual revolution ? TvEO. 



SPEED OP AMERICAN ICE-YACUTS. 

 [10] — With reference to your reply to my query at p. 16, I must 

 confess that while I see the reasoning I had supposed conclusive 

 is not 80 in reality, yet I fail to understand how there can be any 

 driving-force on a vessel driven by the wind when her velocity is 

 equal to that of the wind. It seems to mo the observed fact that 

 an ioe-yncht travels faster than the wind can only be explained by 

 supposing that the wind comes occasionally in blasts much stronger 

 than the average force of the wind. A fierce blast might give an 

 ice-yacht a velocity much greater than tho average velocity of the 

 wind, and owing to the slight frictional resistance, she might retain 

 that velocity with very little loss until the next blast came. 



Dpsilon. 



[U] — An explanation of the great velocity attained by ice-yachts 

 is given in the August number of Scribner's Magazine. It runs as 

 follows : — An ice-yacht makes "the greatest speed on that course 

 in which she meets with the least air friction, receives the strongest 

 push of the wind in a forward direction, and yet does not lose the 

 wind too much by her own speed. This course is running free, with 

 the wind on the quarter, or about one hundred and thirty-five degrees 

 off her course. Suppose the boat heads north-west, while the wind 

 blows from the west. Now, her speed diagonally across the wind 

 causes her to receive the wind on the beam, as if it blew 

 from the north-west. She practically has a wind on tho 

 beam ; this offers but little air friction against her forward 

 motion. Tiio running-friction is so slight that the boat keeps lier 

 Q "way, the direction of the sail- 

 push is sufficiently fonvard to 

 be advantageous, and, lastly, 

 her diagonal coarse, partly 

 with and partly across the 

 wind, saves her from losing 

 too much of the wind's force 

 by her own speed. Suppose 

 that a twenty-knot breeze 

 blows from li to C, and that 

 she heads towards D, while the 

 wind, represented by the arrow 

 A, blows in a given time to C, 

 it carries the boat with it in 

 nearly the same time ; but as 

 she heads diagonally across 

 the wind, she is obliged to 

 run the long distance from B 

 to D, while the wind blows 

 only from B to C ; she there- 

 fore beats the wind. Uor 

 speed is limited only by the 

 loss or change of the wind 

 through the effects of her 

 own velocity. The greatest 

 velocity of an ice-yacht is 

 not recorded, because her finest runs occur either at unexpected 

 moments, or when she sails over unmeasured distances. But the time 

 over short and long courses lias often been taken. The distance 



from New Uamliurg to Poughkoppsie is over seven milnn. The 

 Sn'iirjhde ran this course in seven minutes. This is the quickest 

 time on record ; but many winters the trip has been mode in from 

 nine to ten minutes. This speed is attained with a stiff breeze on 

 tho beam or on tho quarter, and when tho ice is tolerably smooth 

 and clear of im]iasBablc cracks. But an ice-yacht ver)' seldom 

 runs a straight course for even a mile. Various obstructions l^vo 

 to be avoided j the wind changes direction vcrj- often, and also 

 comes and goes in fitful puffs over the hills. The consequence is 

 that she makes a very crooked course at very uneven speed ; she 

 goes more than seven miles, and sails at her full speed daring umch 

 less than seven minutes. Probably she flies at times from eighty to 

 one hundred miles an hour. The speed of an ice-yacht, in working 

 to windward, which is her poorest course, is from ten to fifteen 

 miles an hour, against an eight or ten-knot breeze." 



Yachtsiux. 

 [Thanks to "Yachtsman" for his interesting extract. I do not 

 think the explanation, however, is strictly satisfactory. What wo 

 want to have explained is just what the explanation takes for 

 granted, namely, tuhy the ship runs the longer distance BD, while 

 the wind has gone over the shorter distance EC. Any account 

 which simply says that she does, or that she is obliged to, can 

 hardly be regarded as an explanation. When we consider, too, 

 that with every change ofj her velocity there is a change in the 

 direction in which tho sail-pnsh acts, of which the above so-called 

 explanation takes no account, it must be admitted that it is open, 

 as an explanation, to objection. The explanation referred to by 

 " Upsilon," which I gave in the Kewcastle Weekly Chronicle, ran 

 somewhat on this wise : — 



Lot AB be the centre line of the yacht (bow at B), CD her sail. 

 Let CE represent the yacht's velocity at any moment, the effect of 

 which is, of course, equivalent to the existence of a wind blowing in 

 direction EC, and having its velocity represented by EC. Let the 

 actual wind be in direction FE (which would be, if the yacht were 

 at rest, a wind on her quarter), and represented in velocity by FE. 

 Then, by the principle known as the " triangle of forces," we know- 

 that the effective wind comes in direction FC, and with a velocity 

 represented by FC. Now, we must resolve this into the two velo- 

 cities FG, perpendicular to the sail, and GC parallel to it ; of course 

 the resolved part GC corresponds to a wind which could have no 

 effect whatever, and therefore may be neglected. What remains is a 

 wind blowing with velocity FG square to the sail. This winl wo 

 resolve in turn unto two, one IIG, parallel to the yacht's course, the 

 other FH, perpendicular to it. The last has no effect in urging on 

 tho yacht, only inproducing lee way. The effective driving wind with 

 velocity HG remains, and nnless the accelerating force of this wind 

 is counterbalanced by frictional resistance, it will increase the yacht's 

 velocity, which already exceeds that of the wind. — Ed.] 



ARE MEN'S HEADS SMALLER THAN OF YORE? 



[12] — In a letter to your contemporary Nature, Mr. W. B. 

 Kesteven states that in the hat trade the opinion is prevalent that 

 within the last thirty or forty years the size of men's heads has 

 diminished. " The following statement," he says, " has been given 

 to me by a hatter, whose name has attained a pre-eminence lasting 

 more than a generation." " Five-and-thirty years ago, when I was 

 a young man, we used to purchase hats for retail trade in the fol- 

 lowing ratio : — 



Sizes 21— 211— 22— 221— 23— 23Hnches 



Relative number — 1 — 2 — 4 — 3 — 1 



At the present time," he adds, " I am selling hats in this ratio : — 



Sizes 21-21J—22—221— 23— 23i inches 



Relative number 3— -1 — 3— 1 — 1— 0" 



.\ manufactm-or writes : — " I should say that heads generally are 

 two sizes less than at the time you refer to. A head of more than 

 twenty-four inches' circumference is now quite a rarity, whilst wo 



